Devices such as snubbers have been commonly applied in order to control dynamic excitation of piping systems from loads such as those caused by earthquakes. The intended function of these devices is to allow a pipe to move in order to accommodate normal operating conditions such as thermal movements, but to lock to prevent response to dynamic loading. Typically snubbers are either hydraulic or mechanical devices. Both types have experienced difficulties in service.
Mechanical snubbers often suffer damage to internal components which causes the snubbers to lock up and fail to accommodate free movement of the piping due to normal operational thermal cycling. This introduces the possibility of overloading of the piping and support system and the potential failure of the piping or the support system.
Hydraulic snubbers have the potential of leakage which could prevent them from adequately controlling the dynamic excitation of the piping. There is an additional record keeping complication with hydraulic snubbers because of the non-metallic components therein.
In nuclear power plants the history of failure of mechanical and hydraulic snubbers has led to a requirement for regularly scheduled inspection, testing and maintenance of all snubbers in a plant. These programs represent a significant financial burden to the owners of these power plants.
There have been efforts to provide simpler, more reliable substitute devices for snubbers in order to eliminate the costs of inspection, testing and maintenance programs. Among these approaches is the limit stop or gapped restraint. This type of device provides controlled limits within which the pipe can move freely to provide for normal operational movements but beyond which the pipe comes hard against a stop. It is generally true that when these devices bottom out due to dynamic loading, the impact loads on the device are unacceptably high for the existing building structure.